Workaround for NMake

NMake cannot parse macro expansion in LHS of replacement.

```
fatal error U1005: syntax error : text must follow ':' in macro
```

Author: nobu

common.mk

@@ -85,31 +85,33 @@ MAKE_ENC      = -f $(ENC_MK) V="$(V)" UNICODE_HDR_DIR="$(UNICODE_HDR_DIR)" \
 
 PRISM_BUILD_DIR = prism
 
-PRISM_FILES = prism/api_node.$(OBJEXT) \
-		prism/api_pack.$(OBJEXT) \
-		prism/diagnostic.$(OBJEXT) \
-		prism/encoding.$(OBJEXT) \
-		prism/extension.$(OBJEXT) \
-		prism/node.$(OBJEXT) \
-		prism/options.$(OBJEXT) \
-		prism/pack.$(OBJEXT) \
-		prism/prettyprint.$(OBJEXT) \
-		prism/regexp.$(OBJEXT) \
-		prism/serialize.$(OBJEXT) \
-		prism/static_literals.$(OBJEXT) \
-		prism/token_type.$(OBJEXT) \
-		prism/util/pm_buffer.$(OBJEXT) \
-		prism/util/pm_char.$(OBJEXT) \
-		prism/util/pm_constant_pool.$(OBJEXT) \
-		prism/util/pm_integer.$(OBJEXT) \
-		prism/util/pm_list.$(OBJEXT) \
-		prism/util/pm_memchr.$(OBJEXT) \
-		prism/util/pm_newline_list.$(OBJEXT) \
-		prism/util/pm_string.$(OBJEXT) \
-		prism/util/pm_strncasecmp.$(OBJEXT) \
-		prism/util/pm_strpbrk.$(OBJEXT) \
-		prism/prism.$(OBJEXT) \
-		prism_init.$(OBJEXT)
+PRISM_SOURCES = prism/api_node.c \
+		prism/api_pack.c \
+		prism/diagnostic.c \
+		prism/encoding.c \
+		prism/extension.c \
+		prism/node.c \
+		prism/options.c \
+		prism/pack.c \
+		prism/prettyprint.c \
+		prism/regexp.c \
+		prism/serialize.c \
+		prism/static_literals.c \
+		prism/token_type.c \
+		prism/util/pm_buffer.c \
+		prism/util/pm_char.c \
+		prism/util/pm_constant_pool.c \
+		prism/util/pm_integer.c \
+		prism/util/pm_list.c \
+		prism/util/pm_memchr.c \
+		prism/util/pm_newline_list.c \
+		prism/util/pm_string.c \
+		prism/util/pm_strncasecmp.c \
+		prism/util/pm_strpbrk.c \
+		prism/prism.c \
+		prism_init.c
+
+PRISM_FILES = $(PRISM_SOURCES:.c=.$(OBJEXT))
 
 COMMONOBJS    = \
 		array.$(OBJEXT) \
@@ -361,7 +363,7 @@ programs: $(PROGRAM) $(WPROGRAM) $(arch)-fake.rb
 $(PREP): $(MKFILES)
 
 miniruby$(EXEEXT): config.status $(NORMALMAINOBJ) $(MINIOBJS) $(COMMONOBJS) $(ARCHFILE)
-miniruby$(EXEEXT): $(PRISM_FILES:.$(OBJEXT)=.c) # for BSD make
+miniruby$(EXEEXT): $(PRISM_SOURCES) # for BSD make
 
 objs: $(ALLOBJS)